Amplifiers apply a certain amount of signal gain to an input signal and produce an output signal that is substantially equal to the input signal multiplied by a signal gain if the input signal at an input of the amplifier is within the input specifications of the amplifiers. The signal gain (or simply gain) produced by the amplifiers can be dependent upon amplifier type, operating frequency of the input signal, design decisions made by creators of the amplifiers, and so forth.
Depending upon application, it may not be desirable to have an amplifier produce an output signal that exceeds an expected magnitude. Signals with magnitudes exceeding an expected value can cause surrounding circuitry to not operate properly or may even cause damage to the surrounding circuitry.
Furthermore, a typical amplifier will have a maximum output signal magnitude which it cannot exceed, regardless of the magnitude of the input signal or the gain of the amplifier. The maximum output signal magnitude can be dependent on the device physics of the components used in the amplifier, the design (configuration) of the amplifier, the capabilities of the power supply used to power the amplifier, and so on.
With reference now to FIG. 1, there is shown a diagram illustrating an output signal 100 of an amplifier as a function of time. The output signal 100 of the amplifier can vary with time, depending upon an input signal to the amplifier as well as the amplifier's signal gain. As long as the magnitude of the input signal stays below a maximum magnitude value, the output signal can substantially be a magnified version of the input signal. However, if the magnitude of the input signal exceeds the maximum magnitude value, then the amplifier may not be able to produce an output signal that is simply a magnified version of the input signal. In such situations, the amplifier may simply not have adequate capability to produce the correct output signal corresponding to the input signal.
The diagram shown in FIG. 1 illustrates a situation wherein the output signal 100 tracks an input signal until a point wherein the magnitude of the input signal exceeds the maximum magnitude value (shown in FIG. 1 as line 105, illustrating a magnified version of the maximum input magnitude value). When the magnitude of the input signal exceeds the maximum input magnitude value, the amplifier is not capable of producing a signal at its output that correctly represents the input signal. The amplifier (or circuitry internal (or external) to the amplifier) can limit (clamp) the output signal. The limiting of the output signal can be achieved by limiting either the input signal or the output signal and can be performed very rapidly or gradually. The diagram shown in FIG. 1 illustrates a first limited portion 110 of the output signal that is limited rapidly and a second limited portion 115 of the output signal that is limited gradually. The gradual limiting of the output signal is typically preferred since it helps to reduce the chance of the amplifier becoming unstable.
With reference now to FIG. 2, there is shown a diagram illustrating a technique to limit an output signal of an amplifier 200 by limiting an input signal of the amplifier 200. The amplifier 200 is a differential mode amplifier. The amplifier 200 has two inputs, a first input “A” and a second input “A_bar.” A difference between the first input “A” and the second input “A_bar” represents the input signal. Coupled in between the first input “A” and the second input “A_bar” is a transistor 205, preferably a field-effect transistor (FET), with a source terminal of the transistor 205 being coupled to one of the inputs of the amplifier 200 and a drain terminal of the transistor 205 being coupled to the remaining input of the amplifier 200. A gate terminal of the transistor 205 is coupled to an output of a comparator 210 which can be used to compare an output of the amplifier 200 with a reference voltage “VREF.” The reference voltage “VREF” can be representative of a maximum output level (or an output level close to the maximum output level) of the amplifier 200. Since the output of the amplifier 200 is also a differential signal, the comparator 210 can compare the reference voltage “VREF” with one of the two signals representing the output of the amplifier 200.
While the output of the amplifier 200 is less than the reference voltage “VREF” the transistor 205 is open and the two inputs of the amplifier 200 are decoupled. However, if the output of the amplifier 200 is equal to or greater than the reference voltage “VREF” the transistor 205 is closed and the two inputs of the amplifier 200 are coupled together and the difference between the first input “A” and the second input “A_bar” is reduced, bringing the input signal below the maximum magnitude value. The transistor 205 can effectively behave like a variable resistor, therefore, the difference between the first input “A” and the second input “A_bar” can be reduced gradually (depending on a difference between the output of the amplifier 200 and the reference voltage “VREF,” for example) and produce a gradually limited output signal.
With reference now to FIG. 3, there is shown a diagram illustrating limiting an output signal, wherein the output signal is a differential signal. The diagram shown in FIG. 3 illustrates an output signal comprised of a first signal 305 (shown with a heavy solid black line) and a second signal 310 (shown with a heavy dotted line). The first signal 305 can be thought of as a complement of the second signal 310. If the output signal is centered around zero, then when the first signal 305 is positive valued, then the second signal is negative valued. A pair of dashed lines (dashed line 315 and 320) indicate magnitudes which the output signal should not exceed, either due to maximum signal value limitations of surrounding circuitry or maximum magnitude values of an amplifier used to generate the output signal. When the first signal 305 exceeds the dashed line 315, it is limited and an exemplary limited signal is shown as curve 325. Since the first signal 305 and the second signal 310 are complementary, when the first signal exceeds the dashed line 315, the second signal 310 falls below the dashed line 320 and must also be limited (shown as curve 330).
A prior art technique involves the use of a signal magnitude detecting circuit (or circuits) coupled to a signal output(s) of the amplifier. The signal magnitude detecting circuit can detect when the magnitude of the output signal of the amplifier exceeds the expected magnitude (or a magnitude close to the maximum output magnitude) and when this occurs, the input signal of the amplifier may be clamped to a value so that given the gain of the amplifier, the output signal of the amplifier does not exceed the expected magnitude. The clamping of the input signal can be accomplished by coupling a voltage potential with a given level to the input of the amplifier, for example. The clamping of the input signal can vary from reducing the input signal by just a small amount or reducing the input signal all the way down to zero.
One disadvantage of the prior art is that a sharp change is introduced to the input signal when it is coupled to the voltage potential. The sharp change in the input signal can introduce instability to the amplifier and can result in oscillation and other undesired behavior.
Another disadvantage of the prior art involves where in the amplifier and attendant circuitry the clamping of the input signal occurs. Depending upon where the input signal is clamped, the output signal being fedback may also become clamped.